Polyester, and particularly polyethylene terephthalate, is widely used as a material for production of fibers and films, or as a molding material, and its industrial value is quite high. For such a molding material or industrial fibers, a polyester is required having a high degree of polymerization. As a process for the production of polyester, the batch polymerization method, the continuous polymerization method, and so forth are known as described in Seni Binran (Fiber Handbook), published by Maruzen Co., Ltd., (Nov. 30, 1968). These conventional methods, however, have disadvantages in that a long time is needed for polymerization, and it is quite difficult to produce polyester having an intrinsic viscosity [.eta.] of at least 0.8 dl/g on a commercial scale.
As a method to increase the rate of polymerization of polyester, a method is known in which BHET (bishydroxyethyl terephthalate) or an oligomer thereof is sent to a heating zone in the form of a thin film having a thickness of from 0.026 to 0.3 mm by the use of an endless metallic belt and is subjected to a polycondensation to produce polyester, as described in Japanese Patent Publication No. 5119/73. This method is limited in its ability to shorten the time for polymerization, and requires at least thirty minutes in the production of polyester having such a degree of polymerization that the intrinsic viscosity is not less than 0.8 dl/g.